Modern athletic footwear is a highly refined combination of elements that each perform a specific function or combination of functions directed toward promoting athletic performance. The primary elements of athletic footwear are an upper and a sole. The purpose of the upper is to comfortably enclose and secure the wearer's foot to the footwear while providing ventilation to cool the foot during athletic activities. The sole is attached to the upper and conventionally includes three layers: an outsole, a midsole, and an insole. The outsole forms the ground-contacting layer of the sole and is typically formed of a durable, wear-resistant material. The midsole forms the middle layer of the sole and is formed of a resilient foam material that attenuates impact forces that are generated when the foot contacts the ground. The insole is a thin padded member located within the upper and adjacent to the foot that improves footwear comfort.
In addition to the primary elements discussed above, athletic footwear may incorporate elements that limit pronation or enhance stability, depending upon the activities for which the footwear is designed. Running shoes, for example, commonly incorporate elements that limit the degree and rate of pronation experienced by the foot. Because rapid lateral direction changes, lunges, and jumping are not commonly associated with sprinting or distance running, running shoes often do not incorporate elements that inhibit these motions, thereby facilitating a lightweight article of footwear. In contrast, designs for footwear intended to be worn during court-style activities, including basketball, tennis, and racquetball, incorporate elements that enhance stability during rapid lateral direction changes, lunges, and jumping. Because running is also an important aspect of court-style activities, footwear designed for these sports may also include pronation control elements.
Rapid lateral direction changes, lunges, and jumping have the potential to place high levels of stress upon an athlete's foot. To reduce the probability of injury and improve stability during these motions, it is desirable for the forefoot portion of the foot to rotate with respect to the heel portion of the foot about a longitudinal axis of the foot. That is, it is desirable for the forefoot to be axially decoupled from the heel. In addition, the footwear should be sufficiently flexible in the forefoot portion to permit the digits to bend relative to the foot. Accordingly, footwear for court-style activities, or any other activity that requires a variety of motions, should provide support along the longitudinal length of the foot so as to limit non-axial, vertical flexion in the midfoot and heel area; permit the forefoot to axially flex in relation to the heel; and permit forefoot flexion.
While many sole designs support the foot, they typically do not provide adequate axial flexibility. For example, many midsoles and outsoles are monolithic structures that extend throughout the longitudinal length of the sole. The degree of stiffness in the structures directly correlates with the ability of the sole to longitudinally support a foot. In practice, a sufficiently stiff monolithic sole that fully supports a foot along its longitudinal length also significantly limits the axial flexibility of the shoe.
One known device for supporting the foot, disclosed in U.S. Pat. No. 5,832,634 to Wong, includes a stiffening plate positioned between the midsole and outsole. The stiffening plate is generally planar and constructed of a polymer and a semi-rigid material such as woven carbon fibers or glass fibers that extend longitudinally from a heel portion to a forefoot portion of the sole. The plate improves support and stability of the foot by limiting the flexibility of the sole along an axis transverse to its longitudinal length. Accordingly, the sole remains generally rigid along its length, thereby supporting the entire foot as it rolls from the heel to the toe while running or walking. While a sole having this type of stiffening plate may slightly flex axially about its longitudinal length, the limited degree of axial flexibility may also interfere with the natural pronation of the foot. See also U.S. Pat. No. 4,162,583 to Daria and U.S. Pat. No. 5,845,420 to Buccianti et al.
Soles that include stiffening elements with axial flexibility have been disclosed in, for example, U.S. Pat. No. 4,922,631 to Anderie and U.S. Pat. No. 5,319,866 to Foley et al. The Anderié patent discloses a longitudinal stiffening member positioned along the longitudinal centerline of the sole. The member extends between a front sole portion and a rear sole portion, which are separated by recesses. In the Foley patent, the weight of athletic shoes is reduced by removing a portion of the sole adjacent to a central arch region and replacing it with a lightweight arch support member spanning between an aft heel region and a forefoot region of the sole.
U.S. Pat. No. 5,896,683 to Foxen et al. discloses an article of footwear having a plurality of finger-like elements extending upward from the sole to the upper. The footwear permits flexion in the dorsi and plantar flexion plane, but not in the medial and lateral flexion plane.
Thus, despite the known prior art techniques, there remains a need for a lightweight athletic shoe that provides support along the longitudinal length of the foot so as to limit non-axial, vertical flexion in the midfoot and heel area, while promoting forefoot flexion, and permitting the forefoot to axially flex in relation to the heel.